1. Field of the Invention
The present invention relates to a delay locked loop (DLL) of a semiconductor memory device and, more specifically, to a delay locked loop capable of detecting a change of a clock frequency supplied from a chipset and resetting a phase detector.
2. Discussion of Related Art
Generally, in a semiconductor memory device operating synchronized with an external clock signal, in case where an internal clock signal is delayed for a constant time comparing with the external clock signal, the high frequency operation performance of the semiconductor memory device is deteriorated. Especially, a time when data is outputted after application of the external clock signal, i.e., an output data access time (tAC) is increased. Therefore, in order to prevent the high frequency operation performance of the semiconductor memory device form being deteriorated, a circuit is required to precisely synchronize a phase of the internal clock signal to a phase of the external clock signal, and a delay locked loop is generally used for it. In addition, the delay locked loop is widely used for a clock recovery system, a precise time-to-digital conversion, and high-speed serial links, etc.
Meanwhile, according to an application, one of an analog DLL, a digital DLL and a hybrid DLL may be used. Especially the analog DLL has good jitter characteristics, while it has an essential problem that it is locked in a False State that an internal clock signal is delayed by more than one period with respect to a reference clock signal, for example, an external clock signal. The False State locking is not preferable due to Jitter Accumulation and increased Noise Susceptibility.
Also, the delay locked loop is used to generate clocks having negative delay characteristics of CMOS VLSI circuit and DRAM, etc. The delay locked loop functions to cause an output of data to be coincident with the external clock as the delay locked loop receives the external clock, compensates a delay component of a clock path and a data path, and outputs a signal leading the external clock.
The external clock inputted to a circuit such as DRAM, etc. is applied from a chipset such as a memory controller, and so on. When the frequency of the external clock is varied, the delay locked loop has a high possibility of a malfunction. Thus, there is a problem in that the data output is not synchronized to the clock. FIG. 1 is a block diagram of a conventional delay locked loop.
A delay unit A delays an external clock by a predetermined amount and outputs it.
A replica B is a copy of the clock path and data path, and the negative delay amount of the delay locked loop is determined according to the amount. A phase detector C functions to compare the external clock with the phase of the external clock passing through the delay unit A and the replica B. A clock driver D functions as a driver that supplies the delayed clock delayed in the delay unit A with other circuits.
An operation principle of the delay locked loop of FIG. 1 will be explained in detail below with reference to FIG. 2.
In case where a clock CLOCK is inputted to the delay unit A from an external chip, the delay unit A delays the clock CLOCK and generates a delayed clock CLOCK_D. The delayed clock CLOCK_D is inputted to the replica B. The replica B delays the delayed clock CLOCK_D by a delay amount of the clock path and data path, and generates a clock feedback signal CLOCK_FEEDBACK. The phase detector C sends the delay unit A a signal to increase a delay amount of the delay unit A until a phase of the clock CLOCK is identical to a phase of the clock feedback signal CLOCK_FEEDBACK. Through these processes, when the clock CLOCK and the clock feedback signal CLOCK_FEEDBACK are coincident with each other, the delay unit A is locked and a delay synchronous clock signal DLL_CLK is generated. As a result, it is possible to compensate the delay component of the clock path and data path, and output data DATA synchronized with the external clock CLOCK.
FIG. 3 shows a detailed circuit of the prior art phase detector, and FIG. 4 shows a timing chart for describing the operation thereof.
Initially, a delay amount of the delay unit A is set to “0”, the clock feedback signal CLOCK_FEEDBACK generates a signal lagging by a delay amount of the replica B, comparing with the clock CLOCK. At this time, the phase detector C compares the phase at a rising edge of the clock CLOCK. For example, at a time T1, an output of a NAND gate is low. An output CMP_EN of a NOR gate maintains a high state until a signal passing a delay unit 10 and an inverter G2 becomes high. During the time T1, one output node N1 of a latch 20 maintains the low state and another output node N2 maintains the high state. An output N3 of a NAND gate G3 becomes high and output N4 of NAND gate G4 becomes low. Therefore, an output SHIFT_RIGHT of a latch 30 becomes high. If the output SHIFT_RIGHT is the high state, the delay amount of the delay unit A is increased. If an output SHIFT_LEFT thereof is the high state, the delay amount of the delay unit A is decreased. With repeating these processes, if the phase of clock CLOCK and the phase of the clock feedback signal CLOCK_FEEDBACK are coincident with each other, a phase of a signal DLL_LOCK showing that the delay locked loop is locked becomes high. This situation is the same as the timing chart of FIG. 2, and the clock feedback signal CLOCK_FEEDBACK has a phase lagging by a delay amount D of the delay unit A and a delay amount of the replica B, comparing with the clock CLOCK. Namely, if the delay locked loop is locked, D+T=1T (T: clock period). Therefore, the output of the delay unit A has a phase leading by a delay amount R of the replica B, comparing with the phase of the clock, as shown in FIG. 2.
Typically, if the delay locked loop is locked, in order to be insensitive to a voltage variation of an external power supply, a circuit is installed to perform a limiting function so that the variation of the delay amount does not occur. At this time, if a frequency of the clock CLOCK inputted from the external chipset is abruptly increased, the phase of the clock CLOCK and the phase of the clock feedback signal CLOCK_FEEDBACK, which are initially identical to each other as shown in FIG. 5, are not coincident with each other. Thus, the output data DATA is not coincident with the phase of the clock CLOCK. This problem is continuously maintained before the delay locked loop is reset.
In general, a semiconductor circuit is reset by a power up signal generated when all semiconductor circuits are powered up when the semiconductor circuit is powered up. Namely, since the conventional delay locked loop is a structure reset by both a power up signal PWRUP generated when powering up and a self refresh operation signal, so that, even though, the phase of the clock CLOCK and the phase of the clock feedback signal CLOCK_FEEDBACK, which has initially been coincident with each other do not become coincident by a change of the clock, the delay locked loop can not be reset as long as a power up signal is not newly supplied.